A ferromagnetic body means a material which is spontaneously magnetized even though a strong magnetic field is not applied thereto from the outside. In a magnetic tunnel junction structure (including a first magnetic body, an insulating body and a second magnetic body) in which an insulating body is interposed between two ferromagnetic bodies, a tunnel magneto resistance effect in which an electric resistance varies depending on relative magnetization orientations of two magnetic layers occurs, since up-spin and down-spin electrons flow at different degrees at the magnetic tunnel junction structure while tunneling an insulating body. This tunnel magneto resistance has a greater value than a huge magnetic resistance generated at a spin valve structure (including a first magnetic body, a non-magnetic body and a second magnetic body) in which a non-magnetic body is interposed between two ferromagnetic bodies instead of the insulating body, and thus this is widely used as an essential technique of a magnetic memory device for sensors and information storage in order to rapidly read data recorded on a hard disk.
Due to the tunnel magneto resistance effect, relative magnetization orientations of two magnetic layers control a flow of current. Meanwhile, according the Newton's third law, namely the law of action and reaction, if the magnetization orientation may control a flow of current, it is also possible to control a magnetization orientation of the magnetic layer by applying a current by the reaction. If a current is applied to the magnetic tunnel junction structure in a perpendicular (thickness) orientation, the current spin-polarized by the first magnetic body (the fixed magnetic layer) transfers its spinning angular momentum while passing through the second magnetic body (the free magnetic layer). A torque felt by magnetization due to the transfer of spinning angular momentum is called a spin transfer torque, and it is possible to fabricate a device for reversing magnetization of the free magnetic layer or continuously rotating the free magnetic layer by using the spin transfer torque.
An existing magnetic memory device in which a magnetic tunnel junction structure composed of a magnetic body with in-plane magnetization is applied to a film surface basically has a structure as shown in FIG. 1, which has a structure including an electrode, a first magnetic body (a fixed magnetic layer) 101, an insulating body 102, a second magnetic body (a free magnetic layer) 103 whose magnetization orientation varies due to a current, and an electrode. A transistor is disposed at an upper or lower portion of the magnetic tunnel junction structure, and the transistor plays a role of selectively providing a current flowing in a direction perpendicular to the magnetic tunnel junction located at an upper or lower portion. In FIG. 1, a magnetization orientation of a magnetic body may be aligned as a direction penetrating into the ground or emitting from the ground. Here, the second magnetic body is connected to the electrode so that a flux reversal is induced by a current perpendicularly applied to the film surface. At this time, two electric signals with high and low resistances are implemented according to relative magnetization orientations of the fixed magnetic layer and the free magnetic layer, and a magnetic memory device may be applied to record the above data as “0” or “1”.
If an external magnetic field is used instead of current in order to control magnetization of the free magnetic layer, a half-selected cell problem becomes serious as the device has a smaller size, and thus there is a limit in high density integration of the device. Meanwhile, if a spin transfer torque generated by applying a current is used to the device, flux reversal of the cell may be easily induced regardless of the size of the device. According to the physical instrument of the above spin transfer torque, the intensity of spin transfer torque generated at the free magnetic layer is determined by an amount of applied current density, and thus there exists a critical current density for flux reversal of the free magnetic layer. If both the fixed magnetic layer and the free magnetic layer are composed of material with an in-plane magnetic anisotropy, the critical current density JC may be expressed like Equation 1 below.
                              J          c                =                              α            ⁢                                          2                ⁢                e                            ℏ                        ⁢                                                            M                  s                                ⁢                d                            η                        ⁢                          (                                                H                  K                                +                                                                                                    N                        d                                            ⁢                                              M                        S                                                              -                                          H                                              K                        ⊥                                                                              2                                            )                                =                      α            ⁢                                          2                ⁢                e                            ℏ                        ⁢                                                            M                  S                                ⁢                d                            η                        ⁢                          (                              H                                  K                  ,                  eff                                            )                                                          Equation        ⁢                                  ⁢        1            
In Equation 1, α represents a Gilbert damping constant,  (=1.05×10−34 J·s) is obtained by dividing a Planck constant by 2π, e(=1.6×10−19 C) represents an electron charge amount, η represents a spin polarization efficiency constant determined by the material and entire structure and having a value between 0 and 1, MS represents a saturation magnetization amount of a magnetic body, d represents a thickness of the free magnetic layer, HK represents an in-plane magnetic anisotropy magnetic field of the free magnetic layer, Nd represents an effective demagnetizing field constant and has a value between 0 and 4π depending on the shape of the free magnetic layer when being described in a CGS unit, HK⊥ represents a perpendicular magnetic anisotropy magnetic field of the free magnetic layer, and an in-plane effective anisotropic magnetic field HK,eff of the free magnetic layer is defined as HK,eff=(HK+(NdMS−HK⊥)/2).
If the cell size is reduced to make a highly integrated memory device, a super-paramagnetic limit occurs in which a recorded magnetization orientation is arbitrarily changed due to thermal energy at normal temperature. This may result in undesired deletion of recorded magnetic data. The time t during which a magnetization orientation is averagely maintained against thermal energy may be expressed as Equation 2 below.
                    τ        =                                            τ              0                        ⁢                          exp              ⁡                              (                                  KV                                                            k                      B                                        ⁢                    T                                                  )                                              =                                    τ              0                        ⁢                          exp              ⁡                              (                                                                            H                      K                                        ⁢                                          M                      S                                        ⁢                    V                                                        2                    ⁢                                          k                      B                                        ⁢                    T                                                  )                                                                        Equation        ⁢                                  ⁢        2            
In Equation 2, T0 is a reciprocal of attempt frequency and has a value of about 1 ns, K represents an effective anisotropic energy density (=HKMS/2) of the free magnetic layer, V represents a volume of the device, kB represents a Boltzmann constant (=1.381×10−16 erg/K), and T represents a Kelvin temperature.
Here, KV/kBT is defined as thermal stability Δ of the magnetic memory device. For commercialization as a non-volatile memory, a condition of Δ>50 should be satisfied in general cases. If the volume V of the free magnetic layer is reduced for high density integration of the device, K should be increased to satisfy the condition of Δ>50, and as a result it can be found that Jc also increases. An amount of current provided from a device which applies a current to the magnetic tunnel junction is generally proportional to the size of a transistor connected each magnetic tunnel junction, and this means that the size of the transistor should be greater than a suitable level in order to apply a current density of Jc or above. Therefore, a size of the transistor for applying a current of Jc or above may be a limit in high density integration of the magnetic memory device.
In addition, in the basic structure, if the thickness of the insulating body increases while a current flows through the magnetic tunnel junction, the difference between up-spin and down-spin tunneling electrons becomes greater, and thus the tunnel magneto resistance increases. However, in this case, when the same voltage is applied, the amount of the tunneling current decreases, and thus it becomes very difficult to effectively apply a spin transfer torque for flux reversal to the free magnetic layer. In other words, if the thickness of the insulating body increases, the tunnel magneto resistance also increases which is an essential element in commercialization since a magnetization state may be rapidly read, but it is very difficult to implement a device which satisfies two factors simultaneously since the current density is reduced.